1. Field of the Invention
The present invention relates to a Cr-Mo steel pipe and a welding method thereof and, more particularly, to a steam pipe used for electric power plants or others and the welding method thereof.
2. Description of the Related Art
A 1.25Cr-0.5Mo steel pipe is heavily hardened by welding. Therefore, a preheat treatment and post weld heat treatment are required in welding such type of steel pipes to reduce the hardness and prevent cold cracking in welding and stress corrosion cracking in use. A preheat treatment is needed for preventing the formation of weld cracks. A post weld heat treatment to 500.degree.-700.degree. C. is indispensable for increasing the absorbed energy in a Charpy impact test at room temperature to 10-15 kg.m because the absorbed energy of a 1.25Cr-0.5Mo steel pipe is about 2-3kg.m.
The Quarterly Journal of the Japan Welding Society Vol.3, No.2 issued in 1985 described on its page 371 that a) steel pipes containing C, Mn, Ni, Cr and Mo of certain amounts should be preheated to a temperature below 100 .degree. C. when they are subjected to shielded metal arc welding, b) preheated to a temperature over 225.degree. C. without a post weld heat treatment when subjected to multi-layer shielded metal arc welding, and the interpass temperature should be above 225 .degree. C. Similar facts have been disclosed in Japanese Patent Laid-Open No.56309/1986.
HLPS, stress-relief annealing standard and its explanation describe that a post weld heat treatment is eliminated for pipes with a small wall thickness, when the standard components of SCMV3, STPA23, and STBA23 are preheated to 150.degree. to 300.degree. C. keeping the interpass temperature at 50.degree. to 300 .degree. C.
SCMV3 is stated in JIS (Japanese Industrial Standard) specification referring to a pressure vessel. STPA23 consists of C of 0.17 wt. % or less, Si of 0.44 to 0.86 wt. %, Mn of 0.36 to 0.69 wt. %, P of 0.030 wt. % or less, S of 0.030 wt. % or less, Cr of 0.94 to 1.56 wt. %, Mo of 0.40 to 0.70 wt. %.
STPA23 is a letter symbol of grade for alloy steel pipe stated in JISG3458, which consists of C of 0.15 wt. % or less, Si of 0.50 to 1.00 wt. %, Mn of 0.30 to 0.60 wt. %, P of 0.030 wt. % or less, S of 0.0301.% or less, Cr of 1.00 to 1.50 wt. %, Mo of 0.45 to 0.65 wt. %. STBA23 is a letter symbol of a grade for alloy steel boiler and heat exchanger tubes stated in JISG3462, which consists of C of 0.15 wt. % or less, Si of 0.50 to 1.00 wt. %, Mn of 0.30 to 0.60 wt. %, P of 0.030 wt. % or less, S of 0.030 wt. % or less, Cr of 1.00 to 1.50 wt. %, Mo of 0.45 to 0.65 wt. %.
As described above, 1.25Cr-0.5Mo steel easily develops cold cracking in welding because its heat-affected zone is heavily hardened. To prevent the cracking, it is necessary to perform a preheat treatment to a temperature of 150.degree. to 350.degree. C. to reduce the thermal stress in welding and to decrease the amount of diffusible hydrogen which causes cracking.
In most cases, a post weld heat treatment to a temperature of 600.degree. to 700 .degree. C. is required to soften the heat-affected zone, remove the residual hydrogen, restore the toughness, and prevent stress corrosion cracking. These heat treatments are troublesome and disadvantageous in terms of work cost and work period. Also, they are unfavorable for quality control.